The present invention relates to an attachment lens system for close-up photography used with a master lens in such a fashion that the attachment lens system is attached to the front of the master lens, and more particularly to an attachment lens system for close-up photography suitable for macrophotography at a magnification exceeding unity magnification in the case where the master lens is a macrophotographic lens unit (hereinafter referred to a "macro-lens").
There has been known an attachment lens for close-up photography in use with a master lens in such a fashion that the attachment lens is attached to the front of the master lens. This type of lens, particularly, the attachment lens of low magnification is categorized into two types of lens; a lens consisting of a single lens element and a cemented lens consisting of two lens elements ("smc PENTAX close-up lens S40"), manufactured by ASAHI KOGAKU KOGYO K.K. may typically be enumerated as a single lens element type available on the market and "6.times.7 smc PENTAX close-up lens T132" manufactured by the same company may typically be enumerated as a cemented lens type. The lens of the type in which a macro-lens is used for the master lens, with magnification of 1 to 2 contains typically "smc PENTAX dental close-up lens DS9" for a cemented lens consisting of two lens elements, and the lens disclosed in Japanese Patent Unexamined Publication No. 53-16628 for a cemented lens consisting of four lens elements.
In the single lens element and the cemented lens of two lens elements, a working distance in a condition that rays of light coming from a point infinitely distance are incident on the master lens is small, at most approximately 0.9 to 1.0 times a focal distance of the attachment lens. Accordingly, at some specific magnifications, an object per se casts its shadow on the lens.
There is a lens whose working distance is large, approximately 1.2 times a focal distance of the attachment lens, as disclosed in the above-described publication. This lens is constructed with four groups, each consisting of a single lens element. When comparing with an ordinary lens consisting of one or two lens elements, the instant lens suffers from an increased cost. Further, the second to fourth lens elements of the lens are greatly sensitive. Because of this, an error in the manufacturing stage leads to an amplified degradation of the performance of the resultant lens. Additionally, in macrophotography over unity magnification, it is possible to unsatisfactorily compensate for the lateral chromatic aberration and the coma aberration.